An electronic unit is well-known that operates according to an operation program stored in a non-volatile memory of the electronic unit itself. In this electronic unit, the operation program stored in the non-volatile memory can be rewritten (or updated), using an external unit that is separately prepared, for version upgrade of the program.
In detail, a boot program is previously stored in a non-volatile memory, for communicating with an external unit and downloading a rewrite program (software) for rewriting. As the rewrite program is downloaded using the boot program, the processing sequence is handed over to the rewrite program. The rewrite program retrieves an operation program for rewriting and stores it in the non-volatile memory.
Recently, a processing system having multiple electronic units becomes available. In this processing system, when individual operation programs of the electronic units are rewritten, a boot program of a primary electronic unit of the multiple electronic units is used for downloading the rewrite programs of the primary electronic unit and other secondary electronic units.
However, this processing system includes various conditions. For instance, a given processing system has no secondary electronic unit, while a certain processing system has a secondary electronic unit prohibiting the rewriting of its operation program. Therefore, the boot program stored in the primary electronic unit needs to be prepared and appropriately applied for each of the various conditions.